Independent info for the fishboat addicted.

Feeling Funky: Why boats roll, and what you can do about it

Ever wonder why sailors of centuries past used to yell “Heave-Ho”? It’s because half of them were ready to toss their cookies as soon as the boat left the dock. Back in the olden days when wood ruled the high seas and sails still had a valid purpose, those blow boats rocked back and forth like teeter-totters because they had high metacentric heights. Of course, if you had mentioned this to the tattooed scallywags onboard they might have keelhauled you before stopping to ask what the heck metacentric height is. Why should you care about metacenters, in the first place? Because your boat has one, too—and it has a direct relationship to how often you or your passengers get seasick, and lose lunch all over the deck. This is important because stomach acids can eat away at your gel coat and dull that shiny finish.

Some boats roll, even when it's calm out... and your passengers may feel a little funky.

Rolling Thunder

We already know that no one likes to up-chuck. But, what makes it happen in the first place? The rolling motion of your boat is most often the culprit. Ever notice that most folks are fine as long as you’re running, but as soon as you lay off the throttles, they turn green? It’s because rolling isn’t much of a problem as you shush over the waves at 40-mph; they’re more or less overcome by the phenomenon of planning. But stop the boat, and get ready to rock, baby.

Some boats rock and roll more quickly than others. This is described as their “roll period” and this is where that strange “metacentric” term comes into play. Here’s the deal: metacentric height is defined as “the distance between a point in space on the centerline called the boat’s metacenter, and its center of gravity.” I boil down my interpretation of this definition to “huh?” So let me re-word it in a simpler but no less accurate description: Metacentric height is a measurement of the relationship between a boat’s beam and its center of gravity (COG—the geometric center of the boat’s weight). The bigger the beam and the lower the COG on a boat, the more stability it has at rest and the more quickly it will right itself after beginning to roll, determining the boat’s “roll period.”

On the surface a short roll period seems like good thing, and in many ways it is. The up-chuck factor should be low, and in normal seas comfort levels will be relatively high. Until, that is, a large wave strikes the beam. In this scenario, it’s possible for a boat to be too stable. After heeling over it will want to right itself so quickly that the motion can be violent. The best example of this comes from power cats, which can drop one hull into a trough while the other gets shoved skyward. Then, they both rush in opposite directions to even out with the other. Hence, the term “snap-roll”.

Unfortunately, having too slow a roll period can be just as problematic. If the boat’s tendency to right itself is too slow, once the roll is triggered the boat might just keep on going until it flops over and sinks. Although this phenomenon does not promote seasickness, it’s what we professional mariners generally call “not good.”

Bottom line: the comfiest boats have neither incredibly high nor incredibly low metacentric heights, neither particularly fast nor slow roll periods, but are content in their mediocrity. Just for the record, a powerboat in the 26’ range should commonly have a metacentric height in the range of 4.5, while a big honkin’ 65’ yacht will usually be closer to 10. Think: little boat snappy roll versus big boat slow roll.

How can you prejudge a particular boat’s attitude, when it comes to roll? How will you predict what’s going to happen when you’re trolling through a four foot beam sea? I’d like to hand you off some snappy mathematical formula that answers these questions, but in reality there’s only one thing you can do: take the boat trolling through a four-foot beam sea. Leave all the funky terms and mind-bending equations to the design geeks, and go boating instead. Luckily, if your boat rolls in ways you don’t like you aren’t completely helpless; use these tricks and tools, to reduce the rolling and keep lunch where it belongs.

* Move Your Metacenter – Remember, COG plays a key roll in this equation, and all too often we change our boat’s COG by packing gear into the boat in an off-center position, or by adding weight high in the boat. This problem is particularly evident on small boats, where relatively small weight changes can have dramatic effects. Always consider this when packing gear, and avoid elevating its weight. Some common COG faux pas include placing full coolers or portable livewells atop raised decks, using T-top electronics boxes for gear stowage, and lashing gear to poling platforms.

* Foil The Roll – Adding a hydrofoil to your outboard often reduces rolling notably, sometimes by as much as 15- or 20-percent. Unfortunately, most of the effect will only be evident when running. At rest, the difference will be minimal. News Flash: several new types of hydrofoils can be mounted without drilling into the anti-ventilation plate, greatly simplifying installation.

* Rest, at Rest – Another handy add-on is the Magma Rock ‘n Roll Boat Stabilizer (www.magmaproducts.com). This system simply applies pressure to slow and reduce rolling motions, with a hinged-wing stabilizer and an aluminum outrigger. It’s just like the “birds” hung from the outriggers of commercial fishing boats—remember Billy Tyne, cutting free the swinging bird in The Perfect Storm? Commonly such products made for recreational boats are called “flopper stoppers”. One important difference: Unlike most products of this type, the stainless steel Magma stabilizer is hinged in the middle. When the boat rocks towards the wing it folds in half and sinks with little resistance through the water. When the boat rocks back in the other direction the wing opens, catches some serious water, and reduces the motion of the boat.

I tested the Rock ‘n Roll on a 19’ boat while adrift in 2’ to 3’ seas and was amazed at how big a difference it made. Before deploying it I timed and measured the roll at an average of 3.2 seconds and seven degrees. After putting it over the side the average level of inclination dropped to four degrees and the average roll period went up to 3.4 seconds. Numerically that might not sound like a huge difference, but from my perspective standing on the deck, it was. In my gut, it felt like the seas had dropped by 25-percent. But, there’s a significant draw-back to using the Rock ‘N Roll: It can only be used while drifting or at anchor, since you’ll have to hang a boom and lines over the side of your boat. That also means that anglers will have to deal with extra snags in the water.

* Big Bucks = Big Comfort – If you have cash to burn and are willing to pay between $30,000 and $50,000 to get rid of the roll, you can install a anti-roll gyro in your boat. These systems (there are several on the market, most made by Mitsubishi and Seakeeper,) look like a simple fiberglass box bolted to the stringers. Get inside the cover, however, and you’ll see a flywheel set on gimbals in an oil bath. Once they’ve spooled up (a process that takes about half an hour) these flywheels produce approximately 1,400 ft.-lbs of counter-torque to a boat’s rolling motion.

I experienced the Mitsubishi system in action, on a Bertram 570 Convertible rigged with two ARG 2000’s, the smallest units available. Our test day was ideal, with 15- to 20-knot winds, and 3’ to 5’ seas. We parked the boat beam-to and I measured heel angle, roll period, and the amount of rolls it took the boat to regain a level keel after being struck on the beam by a five-footer. With the system off, virtually all of the 20 sets of waves I measured sent the boat into series of rolls that usually heeled beyond my inclinometer’s range—10-degrees—for two or three rolls. Most of the time, the boat was struck by another large wave set and started off-the-meter rolls again before ever settling on an even keel. With ARG running, however, my inclinometer broke 10-degrees only four times in 20 sets. On the follow-up roll it dropped between 3 and 8 degrees, and roll period dropped from an average 9.8 seconds to an average of 7.1 seconds. On deck the shorter roll period did not feel any more abrupt, or “snappy”, because the rolls were so much smaller than they had been. After punching my test data from 40 wave sets into the calculator, I came up with a 32.75-percent roll reduction. An even bigger reduction was detected by my internal queeze-o-meter—and as any captain who’s been stuck with the job of swabbing a puke-strewn deck can tell you, that’s the meter that really counts.